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Deceleration distance over time

  1. Jan 4, 2017 #1
    1. The problem statement, all variables and given/known data:
    A person drives a car for 20 seconds in a straight line with an initial velocity of 20m/s. During the entire course of the journey they applied breaks causing the car to decelerate at 5m/s2. How far will it be from the starting point after the given time?

    2. Relevant equations:
    distance = 20 * 20 + 0.5 * -5m/s^2 * 20^2


    3. The attempt at a solution:
    However since I acquire a negative value, I'm convinced that my working is incorrect. What am I misunderstanding about the deceleration?
     
    Last edited by a moderator: Jan 4, 2017
  2. jcsd
  3. Jan 4, 2017 #2

    BvU

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    Hello Plebian, :welcome:

    So what happens to the speed of the car ? As a function of time I mean.
     
  4. Jan 4, 2017 #3
    I forgot to mention that I attempted the deceleration over time 20 * 20 + 0.5 * -5m/s^2 * 20 this rendered a better result that looks like it could be correct. I am on the correct track here?
     
  5. Jan 4, 2017 #4

    BvU

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    No ! it's not as if you can solve this by trial and error juggling the expressions. The dimensions don't even fit.
    Now read post #2 and react to it: what happens if you slam the brakes while moving at 20 m/s and thus reduce your speed with 5 m/s per second ?
     
  6. Jan 4, 2017 #5
    The velocity of 20m/s would slow down by 5m/s per second.
     
  7. Jan 4, 2017 #6

    BvU

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    Yes, that's what I wrote. So after 1 second your speed is .., after 2 ..., after 3 .... and after 4 it is zero. Would it then go negative if you keep braking ?
     
  8. Jan 4, 2017 #7
    It would remain stationary at that point.
     
  9. Jan 4, 2017 #8

    BvU

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    Does the expression you use in post #1 reflect that ?
     
  10. Jan 4, 2017 #9
    Not at all, which was why I initially posted this query.
     
  11. Jan 4, 2017 #10

    BvU

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    So for how long can you use that expression ?
     
  12. Jan 4, 2017 #11
    Point taken.
     
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